On the philosophy of plants and plant intelligence

Plant.

For many of us the ideas and images that this word conjures up are of a particular kind. We may imagine a forest, a single tree in a park, a garden, a field of daisies, a handful of picked flowers, a crop. What springs to mind may be a thing that grows beside the road, still and leafy, perhaps gently swaying in the breeze. Relaxing, splashes of colour on a green backdrop, unconcerned if we snap off a small branch, unconcerning as we go about our daily business. We know they aren’t just there to look at and enjoy, though, plants are also very useful; they are sources of food and of medicine for many animals, including us. Throughout history humans have written countless classificatory manuals and amassed thousands of specimen collections that help us identify and describe species for these and other purposes.

Useful plants: Aristotle’s pupil Theophrastus, often called the ‘father of botany’ gave us the first scientific description of citron. This fruit had many medicinal, culinary and other uses in ancient times.

Plants are things we appreciate for their aesthetic, metaphoric, and sensory qualities too – the beauty of a rose, the cheerfulness of a sunflower, the sweet, sharp scent of crushed mint, the alarming rafflesia flower in bloom, larger than a child, spattered with a shock of reds and leopard-spots, emanating a stench of decaying flesh. They feature in our art, have inspired many a poem with their beauty and their shade, with their capacity to create for us respite, healing, calm, delight, a sense of place and memory. For us plants form a large part of the landscape, a beautiful and varied backdrop against which our dramas and activities and those of the rest of the animal kingdom play out. They form for us too a continuously regenerating food source that enables these activities.

For thousands of years in the minds of many people, those involved in agriculture and in the western philosophical tradition and its scientific offshoots, this is the way plants have been viewed. They are the background; they are a resource we can use. Plants form the base of the pyramid of life, or the ‘great chain of being’ upon which we humans sit at the pinnacle. “Plants have the form of life that is least developed,” wrote Thomas Aquinas in the late 1200s, “The highest degree of life, however, is in man.”[i] This perspective is only very recently in the history of ideas, starting to have its foundations forcefully shaken.

There have been many colourful and fascinating folk and philosophical ideas about plants throughout the last few millennia and throughout the world. However, to begin the story about plants that was set to dominate early scientific thinking and continues to shape modern assumptions, we must travel back (as is the case for so many of the stories behind western ideas), to Aristotle. Writing in ancient Greece in the century before 300BC, the view of plants that Aristotle painted set the stage for many centuries of dismissive and exploitative assumptions about plant life to come but also kick-started the development of the science that would eventually begin to unravel these same assumptions.

Plants, according to Aristotle, are basically defective animals. They are like animals in being alive, but only capable of nutrition and growth, not sensation, appetite or movement and certainly not cognition or “rationality”, which he saw as solely the domain of man (though animals needn’t feel too bad – he also viewed this last, rational element of the ‘soul’ as completely lacking in slaves, incomplete in children and defective in women, whose rational faculty ‘lacked authority over their irrational soul’!). His view of the living world was a hierarchical one and his view of plants was strictly utilitarian; “Plants,” Aristotle claimed, “exist for the sake of animals, the brute beasts for the sake of man.”[ii]

For hundreds of years after the ancient Greeks, progress in European botany stalled. Outside of herbals during the early to mid-medieval period, there were almost no original writings or new insights, much misinformation about plants was spread and reproduced and empirical study of the natural world ran up against severe sanctions from the church. However, in the late 1100s Aristotle entered the stage once more. His writings were amongst the many ancient texts reintroduced to the western world from both Islamic sources and the original Greek and were translated and popularised throughout Europe in the early renaissance. Thomas Aquinas in the mid 1200s, in his efforts to reconcile traditional theology with Aristotle’s natural philosophy, resolved many perceived tensions between these approaches and enshrined the general Aristotelian approach to the natural world within the curriculum at institutes of higher education, endorsed by the then-powerful Catholic church.

Didacus Valades’ famous 1579 depiction of the ‘great chain of being’

The influence of Aristotle’s views on the subsequent development of early botany and western science in general cannot be overstated. Ironically, although the reconciliation of natural philosophy with theology meant that science could now be pursued without immediate risk of heresy charges and excommunication from the church, Aristotelian ideas on nature subsequently became enshrined dogma that substantially impeded scientific progress in the studies of plants as well as the rest of the natural world. To quote Bertrand Russel rather dramatically on the topic, “throughout modern times, practically every advance in science, in logic, or in philosophy has had to be made in the teeth of the opposition from Aristotle’s disciples”.[iii]

This can certainly be said of advances in the study of plant behaviour and intelligence. As a representative example, as late as 1911, over two thousand years after Aristotle, philosopher Henri Bergson in his Creative Evolution was still making claims along the old lines: “hence the world of plants with its fixity and insensibility, hence the animals with their mobility and consciousness”.[iv] This pervasive juxtaposition of, on the one hand passive, reflexive, immobile plants and on the other active, intelligent, purposive animals, is an incredibly exaggerated one that within today’s philosophical and general circles we still find bandied about. There are of course many differences between plants and animals but this simplistic contrast does those differences no justice.

Later renaissance thinkers such as Francis Bacon, René Descartes and John Locke challenged many problematic parts of the general Aristotelian philosophy and curriculum. However, although the frameworks and arguments supporting Aristotle’s claims about plants were rejected, the specific assumptions about them remained. Plants’ lack of sensory capacity, lack of active movement and their general backgrounding and status as just a resource rather than purposive organisms in their own right remained a framework that persisted well beyond the renaissance into modern times, despite the mounting body of evidence that suggested it was incorrect. “Just as men lived in the firm belief that human destinies depended upon the stars,” writes the botanist Kerner von Marilaun in the introduction to his 1894 textbook, “so they clung to the notion that everything upon the earth was created for the sake of mankind”.[v]

In the late 1800s most of Europe’s renowned plant physiologists still assumed along the same lines as renaissance thinkers several centuries earlier that plant movement was mechanical and insensitive. Julius von Sachs, for example, was a great German scientist credited with establishing plant physiology as a distinct discipline and pioneering approaches to reliable laboratory methods. For him, phototropism (movement of plants toward light) was the mechanical result of sunlight hitting cells in plant stems, causing osmotic changes in turgor that resulted in an overall bending. He quite scathingly criticised those amongst his peers who suggested that it might instead be a matter of sensation and response.[vi]

Real challenges to this view began in 1880, when Charles Darwin published a book called The Power of Movement in Plants. One of the many things he described in this book involves a now-famous set of ingeniously simple experiments on plant movement in response to various environmental signals. These showed that in plants the shoot tip (in the case of responding to light) and of the root tip (in the case of responding to gravity) is the primary site of sensation, while the parts of the plant that respond are distal, leading him to conclude that there is a “localisation of their sensitiveness, and the transmission of an influence from the excited part to another which consequently moves”.[vii] Darwin’s studies of movement in plants and his conclusions were radical for their time. Initially criticised and rejected by many of his contemporaries (including Sachs [viii]), reproduction of his results and further experimental refinements confirmed his findings and the extension of this research program lead to the discovery of the first plant hormone, auxin.[ix]

One of Charles Darwin’s diagrams of a venus flytrap, a plant which delighted Darwin, who described as ‘one of the most wonderful plants in the world’.

Darwin compared plant responsivity to the effects of stimuli on nerves in animals, and in a previous book on climbing plants had commented that “the power of moving, both spontaneously and from various stimulants, is far more common with plants than is generally supposed to be the case by those who have not attended to the subject”.[x] It was Charles Darwin too, who in 1873, organised the event in which Sir John Burdon-Sanderson (a medical physiologist) undertook the first known recording of a plant action potential. It is well known, now, that neurons in animal nervous systems use action potentials to communicate with one another during information processing. It is not well known outside of the community of plant physiologists that plants also use them.[xi]

Without exhausting the reader with an extensive run-down on the history of plant physiology after Darwin (or the complexities of thought on the matter of plant sensation and movement preceding him, only briefly touched on above), suffice it to say that we are, in this day and age, now well aware that plants certainly are actively monitoring and sensing their environments, receiving input from dozens of biotic and abiotic stimuli, combining and multiplexing these signals to produce complex and flexible behavioural responses according to the context and their needs. Their many movements, tropisms and some of the mechanisms underlying these are starting to be well understood. We know plants use a huge variety of hormones as well as electrical signalling for internal coordination of their behaviour, and a wide repertoire of chemicals for signalling and communication with other plants, animals, fungi and bacteria. We know they can recognise self and non-self, that they can discriminate in perception and response between not only plants of different species but can tell kin from non-kin plants, and cooperate more effectively with plants to which they are closely related. We know they use fungi as underground networks not only to share resources but also pick up useful information from their neighbours. We know that they actively form memories, learn and unlearn new behavioural patterns.[xii]

There are many reasons today why the activities of plants remain at the fringes of our thinking such that we fail to appreciate them for what they are. The legacy of Aristotelian thinking lingers implicitly, the timeframes (relative to the speed of animals) of the majority of plant movements are incredibly slow so as to be almost imperceptible to unassisted human perception as movements at all, we have our own cognitive biases when it comes to ascribing ‘aliveness’ to things less like us, or describing ‘behaviour’ as anything but movement, along with the many other historical and philosophical factors at play. Plants still, for many people, are a passive, largely inert (albeit beautiful) background or a useful resource. Many people are simply not aware of the rich diversity of plant behaviours and lifestyles, the incredible complexity of their sensory systems and the ingenuity of their life strategies.

For others, the inability to meet plants where they are at stems from the human tendency to anthropomorphise other beings or to attribute to them almost magical powers – the avid fans of The Secret Life of Plants who believe they can communicate psychically with their tomatoes and that plants have internal conscious worlds structured much like our own (even after decades of research debunking the claims and methods set out in that book and failure to reproduce their results) offer a modern example of this, though there are also plenty of examples from the mythology and plant-lore between and before Aristotle and Darwin too.[xiii] Fascinating and insightful as these myths and stories are, then as now, this tendency is another reason many theorists shy away from discussing plant intelligence or cognition – they are seen as either crude anthropomorphic metaphors or signs of magical thinking, and they prevent us from appreciating plants as they are rather than as we want them to be.

I strongly believe we need to get away from requiring that plants in their behaving be likeus to be intelligent, and instead appreciate the complexity, adaptability and suitability of their behaviour relative to their own requirements and lifestyles. Notwithstanding the usefulness of some plant/animal comparisons and metaphors, we need to explore plants on their own terms, in their own contexts and environments, as their own beings, rather than as set against animals with their capacities and described in terms of what they lack. We need to stop viewing plants as only a resource to be used by us and appreciate them also as beings with their own intrinsically valuable and interesting lives and goings on, actively pursing strategies relevant to their own wants and needs. We need to stop viewing ourselves, and animals in general, as the benchmark against which the rest of the biological world – plants, fungi, bacteria and everything else – is measured. If we can do this it will stretch our philosophies [xiv], stretch our ideas of what it is to be cognitive, to be intelligent, to be an ‘individual’, to behave, to be alive. Appreciating plants in their own right stretches our own sense of place in the world, too, what it is to be human on this planet, sharing it with the rest of life as we do.

Thinking about plants in this way, looking at what they are, what they are doing, why and how they are doing it, exploring their diversity and their quirks, where they sit in and how they contribute to their ecosystems, looking at how we think about and depict them, how we relate to them and how these forms of relating developed and may need to change, how plants relate to one another and to the world around them – these are fascinating explorations that give me endless delight!

In this blog I hope to share with you some of this delight and some of these perspectives and provide you with some food for thought in an informal way, through musings on related topics, through exploration of plants in art and literature, through short summaries of scientific findings and exciting little factoids about plant behaviour, through descriptions and my own sketches of the seasonal changes in the plants around me, links to other people’s writings on the topic and whatever else ends up becoming a part of the process.

Plant.

Hopefully upon exploring these topics more, this word will gradually grow to conjure up in your mind images and ideas, beings and processes altogether different and far richer that what it did at the outset. This has certainly been the process for me, and I hope that you enjoy the journey as much as I do!

[ii] Aristotle, (1952). Politics, in The Works of Aristotle (Vol. 2), W.D. Ross (Ed.) and B Jowett (Trans.). The University of Chicago, Illinois.
Also see Aristotle (1936). On Plants, in Minor Works of Aristotle, W.S. Hett (Ed., Trans.). Heinemann, London.

[iii] Russell, B. (1972), A History of Western Philosophy, Simon & Schuster, New York.

He is even more scathing in The Scientific Outlook (1949, p.43) where, writing on Darwin, Russell comments that “Like every other innovator of modern times, he had to combat the authority of Aristotle. Aristotle, it should be said, has been one of the great misfortunes of the human race.”

[vii] Darwin, C. (1880). The Power of Movement in Plants. John Murray, London.

[viii] Sachs comments on Darwin’s work are situated in the context of the establishment of a laboratory-based discipline of plant physiologists that viewed the methods of gentlemen naturalists such as Darwin as suspect “Charles Darwin and his son Francis…on the basis of experiments which were unskillfully made and improperly explained, came to the conclusion, as wonderful as it was sensational, that they growing point of the root, like the brain of an animal, dominates the various movements in the root.”

For some excellent reviews of this historical context see:
De Chadarevian, S. (1996). Laboratory science versus country-house experiments. The controversy between Julius Sachs and Charles Darwin. The British Journal for the History of science, 29(1).
Hutschera, U. & Briggs, W.R. (2009). From Charles Darwin’s botanical country-house studies to modern plant biology. Plant Biology, 11(6).

[ix] A comprehensive modern book on the role of plant auxin:
Zažímalová, E., Petrášek, J., & Benková, E. (2014). Auxin and its Role in Plant Development. Springer, Dordrecht.

[xii] For a good introductory and popular book on plant perception see:
Chamovitz, D. (2013). What a Plant Knows: A Field Guide to the Senses. Scientific American, New York.

For more comprehensive reviews see anything by Anthony Trewavas. He popularised the notion of plant intelligence in a 2003 paper, has written many papers since and published a fantastic textbook in 2014, Plant Behaviour and Intelligence.

Monica Gagliano from the University of Western Australia does some fantastic work into plant learning, memory and acoustic emissions and perception – her 2014 paper Experience teaches plants to learn faster and forget slower in environments where it matters is a wonderful study on plant habituation learning.

[xiii] Tompkins, P. & Bird, C. (1973). The Secret Life of Plants: A fascinating account of the physical, emotional, and spiritual relations between plants and man. Harper and Row.

The contentious claims and findings continue to hold sway in popular culture but have been disputed by scientists and have failed tests of repeatability:

Galston, A.W. (1979). The not-so-secret life of plants: In which the historical and experimental myths about emotional communication between animal and vegetable are put to rest. American Scientist, 67(3).

Also see for other examples and discussion: Whippo, C.W. & Hangarter, R.P. (2009). The sensational power of movement in plants: a Darwinian system for studying the evolution of behavior. American Journal of Botany, 96.